Or pay for PoW from mobile, for any event: nostr:note1qqqd3xhsuvz2xnvacr3rva3x85p0xt40rcxwxpvm4uk8xly4tc7s5m8g0d

Yes scammers have botnets, but they'll only use them when it's economically viable. But if it's economically viable to mine PoW for spam, then:

1) it's also profitable for honest miners to sell PoW (e.g. in a marketplace), because there is now economic demand for PoW

2) relays can add a number of cheap defenses that makes spamming exponentially more difficult, even for spammers with botnets: for example, AUTH with PoW (one time per session), then post without PoW